$S$ - Statement: An undifferentiated mass of cells in a culture is called callus.
$R$ - Reason: Auxin and cytokinin are added as growth regulators in the culture medium for callus induction.

• A
  $S$ and $R$ are both true,and $R$ is the correct explanation of $S$.
• B
  $S$ and $R$ are both true,but $R$ is not the correct explanation of $S$.
• C
  $S$ is true and $R$ is false.
• D
  $S$ is false and $R$ is true.